Abstract: OBJECTIVE To explore the anti-tumor effect and mechanism of Xiaoai Jiedu Decoction on colon cancer. METHODS CT26 cells were injected subcutaneously into the right armpit of C57 mice to establish the tumor-bearing model. These mouse were sacrificed after 14 d administration. Mononuclear cells of the peripheral blood were isolated and the distribution of Treg cells were detected by flow cytometry. The expression of Foxp3 protein in tumor tissue was detected by immunohistochemistry method. RESULTS Xiaoai Jiedu Decoction significantly inhibited the proliferation of colon cancer in mice， decreased the distribution of CD4 ⁺ Foxp3 ⁺ Treg cells in peripheral blood and decreased the expression of Foxp3 protein in tumor tissues. CONCLUSION Xiaoai Jiedu Decoction can significantly inhibit the proliferation of colon cancer by decreasing the distribution of Treg cells in peripheral blood and the expression of Foxp3 in the tumor tissues， thus affecting the downstream signal pathway.